High-entropy effect is a novel design strategy to optimize properties and explore novel materials. In this work, (La_1/5Nd_1/5Sm_1/5Ho_1/5Y_1/5)NbO₄ (5RNO) high-entropy microwave dielectric ceramics were successfully prepared in the sintering temperature (S.T.) range of 1210–1290 ℃ via a solid-phase reaction route, and medium-entropy (La_1/3Nd_1/3Sm_1/3)NbO₄ and (La_1/4Nd_1/4Sm_1/4Ho_1/4)NbO₄ (3RNO and 4RNO) ceramics were compared. The effects of the entropy (S) on crystal structure, phase transition, and dielectric performance were evaluated. The entropy increase yields a significant increase in a phase transition temperature (from monoclinic fergusonite to tetragonal scheelite structure). Optimal microwave dielectric properties were achieved in the high-entropy ceramics (5RNO) at the sintering temperature of 1270 ℃ for 4 h with a relative density of 98.2% and microwave dielectric properties of dielectric permittirity (ε_r) = 19.48, quality factor (Q×f) = 47,770 GHz, and resonant frequency temperature coefficient (τ_f) = –13.50 ppm/℃. This work opens an avenue for the exploration of novel microwave dielectric material and property optimization via entropy engineering.

AB₂O₄-type spinels with low relative permittivity (ε_r) and high quality factor (Q × f) are crucial to high-speed signal propagation systems. In this work, Zn²⁺/Ge⁴⁺ co-doping to substitute Ga³⁺ in ZnGa₂O₄ was designed to lower the sintering temperature and adjust the thermal stability of resonance frequency simultaneously. Zn_1+xGa_2-2xGe_xO₄ (0.1 ≤ x ≤ 0.5) ceramics were synthesised by the conventional solid-state method. Zn²⁺/Ge⁴⁺ co-substitution induced minimal variation in the macroscopical spinel structure, which effectively lowered the sintering temperature from 1385 to 1250 ℃. All compositions crystallized in a normal spinel structure and exhibited dense microstructures and excellent microwave dielectric properties. The compositional dependent quality factor was related to the microstructural variation, being confirmed by Raman features. A composition with x = 0.3 shows the best dielectric properties with ε_r ≈ 10.09, Q × f ≈ 112,700 THz, and τ_f ≈ -75.6 ppm/℃. The negative τ_f value was further adjusted to be near-zero through the formation of composite ceramics with TiO₂.

A melilite Ba₂CuGe₂O₇ ceramic was characterized by low sintering temperature and moderate microwave dielectric properties. Sintered at 960 ℃, the Ba₂CuGe₂O₇ ceramic had a high relative density 97%, a low relative permittivity (ε_r) 9.43, a quality factor (Q×f) of 20,000 GHz, and a temperature coefficient of resonance frequency (τ_f) -76 ppm/℃. To get a deep understanding of the relationship between composition, structure, and dielectric performances, magnesium substitution for copper in Ba₂CuGe₂O₇ was conducted. Influences of magnesium doping on the sintering behavior, crystal structure, and microwave dielectric properties were studied. Mg doping in Ba₂CuGe₂O₇ caused negligible changes in the macroscopic crystal structure, grain morphology, and size distribution, while induced visible variation in the local structure as revealed by Raman analysis. Microwave dielectric properties exhibit a remarkable dependence on composition. On increasing the magnesium content, the relative permittivity featured a continuous decrease, while both the quality factor and the temperature coefficient of resonance frequency increased monotonously. Such variations in dielectric performances were clarified in terms of the polarizability, packing fraction, and band valence theory.